1. Field of the Invention
The present invention relates to an antenna system mounted on a radio unit such as a portable telephone, and to a radio unit using this antenna system.
2. Description of the Related Art
FIG. 4 is a schematic perspective view showing an example of a surface-mount type antenna. The surface-mount type antenna 1 shown in FIG. 1 has a rectangular-parallelepiped shaped substrate 2 which is formed of a dielectric body. A radiation electrode 3 is formed on the top surface 2a of this substrate 2. The left end side (shown in FIG. 4) of the radiation electrode 3 is extended to a side surface 2b of the substrate 2, and the left end 3a of the radiation electrode 3 forms an open (unconnected) end. On a side surface 2c of the substrate 2, a feeding electrode 4 is extended from the bottom side toward the upper side, and is connected continuously to the right end portion of the radiation electrode 3, wrapping around the top surface 2a of the substrate 2.
Such a surface-mount type antenna 1 is mounted on a circuit board, using the bottom surface 2d of the substrate 2 as a mounting surface, and is conductively connected to a radio unit circuitry 5 formed on the circuit board, via the feeding electrode 4. With the radiation electrode 3 and the radio unit circuitry 5 thus conductively connected, for example, when a signal is supplied by the radio unit circuitry 5 to the radiation electrode 3 through the feeding electrode 4, the radiation electrode 3 is excited by the signal, and the signal (radio wave) is transmitted to the outside. On the other hand, when a signal is received, the signal flows in the direction opposite to that during the above-described transmission, and the received signal is supplied to the radio unit 5. In this manner, the surface-mount type antenna 1 performs an antenna operation of transmitting/receiving the signal (radio wave).
Meanwhile, the frequency band is determined by the effective line length of the signal of the radiation electrode 3. Conversely, the effective line length of the signal of the radiation electrode 3 is determined in accordance with the frequency band of the radio wave transmission/reception based upon a given specification. With the miniaturization of radio units, there is a need also to miniaturize surface-mount type antennas 1 as described above. As the frequency band of the radio wave transmission/reception required for the surface-mount type antennas 1 becomes low, the effective line length of the signal of the radiation electrode 3 must be set to be long, but it can be difficult to ensure a long effective line length in order to perform radio wave transmission/reception in a desirable low frequency band, due to the miniaturization of the surface-mount type antennas 1.
Accordingly, an antenna system 7 as shown in FIG. 5 has been proposed. In FIG. 5, the antenna system 7 is shown as being mounted on a circuit board 11 of a radio unit 10 such as a portable telephone. The antenna system 7 shown in FIG. 5, comprises a surface-mount type antenna 1 as described above which constitutes a surface-mount type antennas portion, and a metallic plate 8 which is conductively connected to the radiation electrode 3 of the surface-mount type antenna 1. In this antenna system 7, by conductively connecting the metallic plate 8 to the radiation electrode 3, the effective line length of the signal of the radiation electrode 3 is equivalently elongated by the increment of the installed metallic plate 8. This facilitates the transmission/reception of radio waves at a required low frequency.
The antenna system 7 is mounted on a circuit board 11 of a radio unit, for example, in an installation configuration as shown in FIG. 5. Specifically, radio unit circuits (5) such as a signal processing circuit and a controlling circuit, are formed on the circuit board 11, and a shield case 13 for shielding these radio unit circuits is provided on this circuit board 11. The top surface of the shield case 13 is parallel with the surface of the circuit board 11 and the top surface 2a and the bottom surface 2d of the substrate 2 of the surface-mount type antenna 1. The surface-mount type antenna 1 of the antenna system 7 is mounted on the circuit board 11 in the vicinity of the shield case 13, using the bottom surface 2d of the substrate 2 of the surface-mount type antenna 1 as a mounting surface. The metallic plate 8 of the antenna system 7 is disposed above the shield case 13 with a space therebetween so that the surface of the metallic plate 8 is parallel with the top surface of the shield case 13.
The circuit board 11 on which the antenna system 7 and the radio unit circuits are provided, are housed in, for example, a case 12 as shown by a chain line in FIG. 5. When the radio unit 10 is a portable telephone, a battery for power in the radio unit is disposed on the blank portion on the circuit board 11 as shown in FIG. 5.
Meanwhile, there is now increased demand for a radio unit 10 such as a portable telephone which has a reduced thickness D. In order to reduce the thickness the radio unit 10, the spacing between the metallic plate 8 of the antenna system 7 and the top surface of the shield case 13 must be made very narrow. However, the entire surface of the metallic plate 8 is opposed to the top surface of the shield case 13 with a space therewith, and the shield case 13 and the surface of the circuit board 11 are equivalent to the ground, and hence, by reducing the spacing between the metallic plate 8 and the shield case 13 as described above, the electrostatic capacitance between the metallic plate 8 and the shield case 13, in other words, the electrostatic capacitance between the metallic plate 8 and the ground, significantly increases. This increase in the capacitance causes problems such as a reduction in the antenna gain, and the narrowing of the frequency bandwidth of radio wave transmission/reception.
It is an object of the present invention to solve the above-described problems. It is, furthermore, an object of the invention to provide an antenna system capable of meeting the demands for the transmission/reception of lower frequency radio waves, and capable of reducing the thickness of a radio unit without deteriorating antenna performance, and also to provide a radio unit using this antenna system.
In order to achieve the above-described objects, the present invention provides, in a first aspect, a surface-mount type antenna which comprises a surface-mount type antenna portion which is configured by forming, on a substrate, a radiation electrode for radio wave transmission/reception, and a metallic plate which is conductively connected to the radiation electrode of the surface-mount type antenna portion, and which performs radio wave transmission/reception together with the radiation electrode. The surface-mount type antenna portion is mounted on a circuit board of a radio unit, using the bottom surface of the substrate as a mounting surface, and the metallic plate is provided so as to be isolated from the circuit board. The metallic plate is preferably disposed adjacent to the long-side surface of the substrate of the surface-mount type antenna portion with a space therebetween so that the surface of the metallic plate is non-parallel with the bottom surface of the surface-mount type antenna portion.
The present invention provides, in a second aspect, a radio unit which includes an antenna system in accordance with the above-described first aspect.
In the radio unit in accordance with the second aspect, it is preferable that the metallic plate of the antenna system be disposed so that the surface thereof is opposed to the corresponding end surface of a circuit board with a spacing therebetween.
In the present invention having the above-described features, the metallic plate of the antenna system is disposed adjacent to the outer long-side surface of the substrate of the surface-mount type antenna portion with a space therebetween so that the surface of the metallic plate is non-parallel with the bottom surface of the surface-mount type antenna portion. When mounting the surface-mount type antenna portion on a circuit board of a radio unit, using the bottom surface of the substrate as a mounting surface, the metallic plate becomes non-parallel with the surface of the circuit board surface, which is equivalent to ground.
Thereby, the occurrence of a large electrostatic capacitance between the metallic plate and the circuit board (that is, ground) can be prevented, and hence, the problem of the increase in electrostatic capacitance between the metallic plate and the ground can be avoided, the increase in the electrostatic capacitance being due to the reduction in thickness of the radio unit. This facilitates achieving a reduction in thickness of the radio unit while preventing the deterioration of the antenna performance.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.